There exist camera systems, such as the Axis Q6000-E series, which integrate one or more fixed sensors with a movable, pan, tilt, zoom (PTZ) camera. The fixed sensors may typically be arranged to provide an overview of a scene, such as a complete 360° field of view, whereas the PTZ camera may be arranged to be directed to and zoom in on specific portions of the scene. In such a system, a user may, for instance, indicate a specific portion of the scene towards which the PTZ camera should be directed by clicking in an overview image provided by one of the fixed sensors. Following such a click, the PTZ camera will be directed to and/or zoom in on the specific portion of the scene.
The viewing direction (i.e., the pan and tilt settings) and/or the zoom of the PTZ camera may thus be controlled by clicking in the images captured by the fixed sensors. Such control of the PTZ camera may rely on a relation between positions in the images captured by the one or more fixed sensors on the one hand, and directions of the PTZ camera on the other hand. Typically, the required relation may be determined from the geometry and optics of the camera system, including the relative positions of the fixed sensors and the PTZ camera, once the direction of the PTZ camera has been initially calibrated with respect to the fixed sensors. More specifically, as the PTZ camera is installed in the camera system, its viewing direction in relation to those of the fixed sensors is not known, and it therefore needs to be calibrated. Such calibration typically aims at finding a correlation between a position in an image captured by one of the fixed sensors and a direction of the PTZ camera.
The patent application published as SG 191452 A1 describes a feature-matching approach for calibrating a PTZ camera with respect to a wide field of view camera. In particular, a method for determining a correlation between a coordinate of an image captured by the wide field of view camera and PTZ-values of the PTZ camera is provided. The precision of the method of SG 191452 A1 relies on the accuracy of the matching of features in the wide field of view image and in an overlapping image captured by the PTZ camera image. The accuracy of the feature matching may in turn depend on several factors, such as the number of features being present in the images and properties of the lenses of the cameras. In the former example, it may happen that the PTZ camera is directed towards a portion of the scene where few objects are present, thereby resulting in few relevant features in the images. In the latter example, it may happen that, due to barrel distortion or other geometric distortions in the wide field of view image, it is difficult to accurately match the features. In both of these examples, the precision of the calibration may in the end be suffering. There is therefore need for improvements.